The announcement adds Boeing and the start-up Karem to the list of previously announced teams selected to compete over the next 20 months.

DARPA plans to select one of the four designs to build a technology demonstrator and perform flight tests by 2017-18, the agency says.

The bidders face a tricky set of requirements. DARPA wants to demonstrate a 4.4t-5.5t (10,000-12,000lb) gross weight aircraft that can cruise above 300-400kt (556-741km/h). It should also have a lift-to-drag ratio of at least 10, twice the aerodynamic efficiency of current VTOL aircraft. DARPA also wants the design to offer a 25% improvement in hover efficiency.

The DARPA requirements also call for the aircraft to carry a useful load of at least 40% of its gross weight, including a payload weighing 15.5% of gross weight.

All four winning teams have based their designs on unmanned aircraft, but that’s where the similarities end.

Among the competitors only Aurora Flight Sciences has not revealed the design but only the branding – LightningStrike. In a 4 February news release about the VTOL X-Plane, the Virginia-based research company and aerostructures manufacturer noted it has been a pioneer in ducted fan and hybrid propulsion aircraft.

The other small business – Karem Aircraft – also has a proud legacy. The company is founded by Abe Karem, inventor of the Predator and Hummingbird unmanned air vehicles. More recently, Karem has proposed a new kind of tiltrotor aircraft with an optimum speed rotor. It can reduce rotor speeds to be more efficient in cruise flight, when the rotor offloads lifting power to the wing.

Meanwhile, DARPA also selected two large companies to compete for the X-Plane project, including Boeing and Sikorsky, which is teamed with Lockheed Martin’s Skunk Works unit.

The Boeing PhantomSwift design embeds twin lifting fans inside the fuselage, with tilting ducted fans mounted on wingtips to provide lift and forward thrust. Last year, Boeing built and flew a scaled-down version of the aircraft.

Sikorsky has described its concept as integrating fixed wing aerodynamics and advanced rotor control in a “low complexity configuration”.

DARPA plans to select a single design in late 2015 to proceed into phase 2, where the VTOL X-Plane will be assembled over an 18-month period.